Transversely-excited film bulk acoustic resonators with piezoelectric diaphragm supported by piezoelectric substrate

ABSTRACT

Acoustic resonators and filter devices, and methods for making acoustic resonators and filter devices. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces. The back surface is attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. A conductor pattern formed is formed on the front surface of the piezoelectric plate, including an interdigital transducer (IDT) with interleaved fingers of the IDT on the diaphragm. An insulating layer is formed between the piezoelectric plate and portions of the conductor pattern other than the interleaved fingers.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application63/091,552, filed Oct. 14, 2020, entitled XBAR WITH INSULATING LAYERBENEATH CONDUCTORS, which is incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 uses the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) formed on a thin floating layer, or diaphragm, of asingle-crystal piezoelectric material. The IDT includes a first set ofparallel fingers, extending from a first busbar and a second set ofparallel fingers extending from a second busbar. The first and secondsets of parallel fingers are interleaved. A microwave signal applied tothe IDT excites a shear primary acoustic wave in the piezoelectricdiaphragm. XBAR resonators provide very high electromechanical couplingand high frequency capability. XBAR resonators may be used in a varietyof RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are well suited for use in filtersfor communications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view and two schematic cross-sectional viewsof a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3 is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 4 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 5 is a schematic block diagram of a filter using XBARs.

FIG. 6 is a chart of the absolute value of admittance versus frequencyfor devices with and without an insulating layer.

FIG. 7 is a flow chart of a process for fabricating an XBAR or a filterincluding XBARs.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are well suited for use infilters for communications bands with frequencies above 3 GHz.

The XBAR 100 includes a piezoelectric plate 110 having a front surface112 and a back surface 114. The front and back surfaces are essentiallyparallel. “Essentially parallel” means parallel to the extent possiblewithin normal manufacturing tolerances. The piezoelectric plate is athin single-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. XBARs may be fabricatedon piezoelectric plates with various crystallographic orientationsincluding Z-cut, rotated Z-cut, and rotated Y-cut.

A surface 122 of a substrate 120 is attached to a back surface 114 ofthe piezoelectric plate 110. A portion of the piezoelectric plate 110 isnot attached to the substrate 120 and forms a diaphragm 115 spanning acavity 140 formed in the substrate 120. The portion of the piezoelectricplate that spans the cavity is referred to herein as the “diaphragm” dueto its physical resemblance to the diaphragm of a microphone. As shownin FIG. 1, the diaphragm 115 is contiguous with the rest of thepiezoelectric plate 110 around all of a perimeter 145 of the cavity 140.In this context, “contiguous” means “continuously connected without anyintervening item”. The substrate 120 provides mechanical support to thepiezoelectric plate 110. The piezoelectric plate 110 may be attacheddirectly to the substrate 120 or may be attached to the substrate 120via one or more intermediate material layers.

The substrate 120 may be, for example, silicon, sapphire, quartz, orsome other material or combination of materials. The back surface 114 ofthe piezoelectric plate 110 may be bonded to the substrate 120 using awafer bonding process. Alternatively, the piezoelectric plate 110 may begrown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1). The back surface 114of the piezoelectric plate 110 and the surface of the substrate 120 maybe attached using a wafer bonding process. Alternatively, thepiezoelectric plate 110 may be grown the substrate 120.

The cavity 140 is an empty space within a solid body of the resonator100. The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 (as shown subsequently in FIG. 3). The cavity 140 may be formed, forexample, by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the portion 115 of thepiezoelectric plate that spans, or is suspended over, the cavity 140. Asshown in FIG. 1, the cavity 140 has a rectangular shape with an extentgreater than the aperture AP and length L of the IDT 130. A cavity of anXBAR may have a different shape, such as a regular or irregular polygon.The cavity of an XBAR may more or fewer than four sides, which may bestraight or curved.

The IDT fingers 136 may be one or more layers of aluminum, an aluminumalloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum orsome other conductive material. The IDT fingers are considered to be“substantially aluminum” if they are formed from aluminum or an alloycomprising at least 50% aluminum. The IDT fingers are considered to be“substantially copper” if they are formed from copper or an alloycomprising at least 50% copper. The IDT fingers are considered to be“substantially molybdenum” if they are formed from molybdenum or analloy comprising at least 50% molybdenum. Thin (relative to the totalthickness of the conductors) layers of other metals, such as chromium ortitanium, may be formed under and/or over and/or as layers within thefingers to improve adhesion between the fingers and the piezoelectricplate 110 and/or to passivate or encapsulate the fingers and/or toimprove power handling. The busbars (132, 134 in FIG. 1) of the IDT maybe made of the same or different materials as the fingers.

An insulating layer 160, as shown in Section B-B of FIG. 1, can bebetween the busbars 132, 134, and the piezoelectric plate to reduceacoustic coupling between the busbars 132, 134 and the piezoelectricplate 110. The insulating layer 160 shown in Section B-B does not extendacross the cavity 140. The insulating layer can be between portions ofthe conductor pattern other than the IDT fingers and the piezoelectricplate. A thickness of the insulating layer can be any suitablethickness. For example, the thickness can be in a range from 1 nm to athickness of the conductor pattern. The insulating layer can be formedof one or more layers, which can be the same or different materials,including, but not limited to SiO₂, Si₃N₄, or other dielectricmaterials.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device willhave an IDT with more than ten parallel fingers. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT. Similarly,the thickness of all elements is greatly exaggerated in the in thecross-sectional views.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The portion of the XBAR 100 shown in FIG. 2 is identified in FIG. 1 as“DETAIL C”. The piezoelectric plate 110 is a single-crystal layer ofpiezoelectrical material having a thickness ts. ts may be, for example,100 nm to 1500 nm. When used in filters for bands from 3.4 GHZ to 6 GHz(e.g. bands n77, n79, 5 GHz Wi-Fi™, 6 GHz Wi-Fi™,), the thickness ts maybe, for example, 200 nm to 1000 nm.

A front-side dielectric layer 214 may be formed on the front side of thepiezoelectric plate 110. The “front side” of the XBAR is the surfacefacing away from the substrate. The front-side dielectric layer 214 hasa thickness tfd. The front-side dielectric layer 214 is formed betweenthe IDT fingers 136. Although not shown in FIG. 2, the front-sidedielectric layer 214 may also be deposited over the IDT fingers 136. Aback-side dielectric layer 216 may be formed on the back side of thepiezoelectric plate 110. The back-side dielectric layer 216 has athickness tbd. The front-side and back-side dielectric layers 214, 216may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfdand tbd are typically less than the thickness ts of the piezoelectricplate. tfd and tbd are not necessarily equal, and the front-side andback-side dielectric layers 214, 216 are not necessarily the samematerial. Either or both of the front-side and back-side dielectriclayers 214, 216 may be formed of multiple layers of two or morematerials.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The geometry of the IDT of an XBAR differs substantially fromthe IDTs used in surface acoustic wave (SAW) resonators. In a SAWresonator, the pitch of the IDT is one-half of the acoustic wavelengthat the resonance frequency. Additionally, the mark-to-pitch ratio of aSAW resonator IDT is typically close to 0.5 (i.e. the mark or fingerwidth is about one-fourth of the acoustic wavelength at resonance). Inan XBAR, the pitch p of the IDT is typically 2 to 20 times the width wof the fingers. In addition, the pitch p of the IDT is typically 2 to 20times the thickness is of the piezoelectric plate 212. The width of theIDT fingers in an XBAR is not constrained to be near one-fourth of theacoustic wavelength at resonance. For example, the width of XBAR IDTfingers may be 500 nm or greater, such that the IDT can be readilyfabricated using optical lithography. The thickness tm of the IDTfingers may be from 100 nm to about equal to the width w. The thicknessof the busbars (132, 134 in FIG. 1) of the IDT may be the same as, orgreater than, the thickness tm of the IDT fingers.

FIG. 3 is an alternative cross-sectional view of an XBAR 300 along thesection plane B-B defined in FIG. 1. In FIG. 3, a piezoelectric plate310 is attached to a substrate 320. A portion of the piezoelectric plate310 forms a diaphragm 315 spanning a cavity 340 in the substrate. Thecavity 340 does not fully penetrate the substrate 320. Fingers of an IDTare disposed on the diaphragm 315. An insulating layer 360 is betweenbusbars 332, 334, of the IDT and the piezoelectric plate 310. The cavity340 may be formed, for example, by etching the substrate 320 beforeattaching the piezoelectric plate 310. Alternatively, the cavity 340 maybe formed by etching the substrate 320 with a selective etchant thatreaches the substrate through one or more openings (not shown) providedin the piezoelectric plate 310. In this case, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around a largeportion of a perimeter of the cavity 340. For example, the diaphragm 315may be contiguous with the rest of the piezoelectric plate 310 around atleast 50% of the perimeter of the cavity 340.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. A radio frequency (RF) voltage is applied to the interleavedfingers 430. This voltage creates a time-varying electric field betweenthe fingers. The direction of the electric field is primarily lateral,or parallel to the surface of the piezoelectric plate 410, as indicatedby the arrows labeled “electric field”. Since the dielectric constant ofthe piezoelectric plate is significantly higher than the surroundingair, the electric field is highly concentrated in the plate relative tothe air. The lateral electric field introduces shear deformation, andthus strongly excites a shear-mode acoustic mode, in the piezoelectricplate 410. Shear deformation is deformation in which parallel planes ina material remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of atomic motion. The degreeof atomic motion, as well as the thickness of the piezoelectric plate410, have been greatly exaggerated for ease of visualization. While theatomic motions are predominantly lateral (i.e. horizontal as shown inFIG. 4), the direction of acoustic energy flow of the excited primaryshear acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 5 is a schematic circuit diagram and layout for a high frequencyband-pass filter 500 using XBARs. The filter 500 has a conventionalladder filter architecture including four series resonators 510A, 510B,510C, 510D and three shunt resonators 520A, 520B, 520C. The seriesresonators 510A, 510B, 510C, and 510D are connected in series between afirst port and a second port (hence the term “series resonator”). InFIG. 5, the first and second ports are labeled “In” and “Out”,respectively. However, the filter 500 is bidirectional and either portmay serve as the input or output of the filter. The shunt resonators520A, 520B, 520C are connected from nodes between the series resonatorsto ground. A filter may contain additional reactive components, such asinductors, not shown in FIG. 5. All the shunt resonators and seriesresonators are XBARs. The inclusion of four series and three shuntresonators is exemplary. A filter may have more or fewer than seventotal resonators, more or fewer than four series resonators, and more orfewer than three shunt resonators. Typically, all of the seriesresonators are connected in series between an input and an output of thefilter. All of the shunt resonators are typically connected betweenground and the input, the output, or a node between two seriesresonators.

In the exemplary filter 500, the series resonators 510A, B, C, D and theshunt resonators 520A, B, D of the filter 500 are formed on a singleplate 530 of piezoelectric material bonded to a silicon substrate (notvisible). Each resonator includes a respective IDT (not shown), with atleast the fingers of the IDT disposed over a cavity in the substrate. Inthis and similar contexts, the term “respective” means “relating thingseach to each”, which is to say with a one-to-one correspondence.

Each of the resonators 510A, 510B, 510C, 510D, 520A, 520B, 520C in thefilter 500 has a resonance where the admittance of the resonator is veryhigh and an anti-resonance where the admittance of the resonator is verylow. The resonance and anti-resonance occur at a resonance frequency andan anti-resonance frequency, respectively, which may be the same ordifferent for the various resonators in the filter 500. Inover-simplified terms, each resonator can be considered a short-circuitat its resonance frequency and an open circuit at its anti-resonancefrequency. The input-output transfer function will be near zero at theresonance frequencies of the shunt resonators and at the anti-resonancefrequencies of the series resonators. In a typical filter, the resonancefrequencies of the shunt resonators are positioned below the lower edgeof the filter's passband and the anti-resonance frequencies of theseries resonators are position above the upper edge of the passband.

Various conductors in a filter may have sufficient voltage differencesbetween them to excite shear waves in the piezoelectric plate betweenthe resonators that can cause spurious modes and excessive loss. Theseshear waves can travel into the substrate and reflect from the back ofthe substrate resulting in multiple resonances. For example, a voltagedifference between a busbar of one XBAR and a busbar of another XBAR cangenerate these undesirable acoustic waves. While the effect of thespurious modes may be mitigated by roughening surfaces of the substrate120, causing acoustic energy to incoherently scatter, it is preferableto avoid the generation of these acoustic waves.

FIG. 6 is a chart 600 of absolute value of admittance as a function offrequency for simulations of two parallel conductors on a surface of apiezoelectric plate supported by a substrate. The parallel conductorsare representative of signal and ground conductors (e.g., busbars ofXBARs) in a filter. Curve 610 shows values for two parallel conductorswith no insulating layer, indicating strong acoustic features andrelatively high admittance. Curve 620 shows values for two parallelconductors similarly configured but with the addition of an insulatinglayer between the conductors and the piezoelectric plate. The admittanceand magnitude of the acoustic features are decreased in curve 620, ascompared to curve 610, due to the presence of the insulating layerbetween the piezoelectric plate and the conductors.

Description of Methods

FIG. 7 is a simplified flow chart showing a process 700 for making anXBAR or a filter incorporating XBARs. The process 700 starts at 705 witha substrate and a plate of piezoelectric material and ends at 795 with acompleted XBAR or filter. The flow chart of FIG. 7 includes only majorprocess steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 7.

The flow chart of FIG. 7 captures three variations of the process 700for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 710A, 710B, or 710C.Only one of these steps is performed in each of the three variations ofthe process 700.

The piezoelectric plate may be, for example, lithium niobate or lithiumtantalate, and may be Z-cut, rotated Z-cut, rotated YX-cut, or someother cut. The substrate may preferably be silicon. The substrate may besome other material that allows formation of deep cavities by etching orother processing.

In one variation of the process 700, one or more cavities are formed inthe substrate at 710A, before the piezoelectric plate is bonded to thesubstrate at 720. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 710A will not penetrate through the substrate.

At 720, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

At 725, an insulating layer is formed on portions or all of thepiezoelectric plate. The insulating layer can be between portions of theconductor pattern other than between the IDT fingers and thepiezoelectric plate. For example, the insulating layer 725 can be formedon portions that correspond to a subsequent position of busbars of IDTsand conductors interconnecting the IDTs. In another example, theinsulating layer can be formed over all of the piezoelectric plateexcept on a diaphragm. The insulating layer can be formed of any one ormore suitable dielectric materials, such as SiO₂ or Si₃N₄. Theinsulating layer can be formed by any suitable method, such as beingpatterned using a mask and etching.

A conductor pattern, including IDT fingers and busbars of each XBAR, isformed at 730 by depositing and patterning one or more conductor layerson the front side of the piezoelectric plate. The conductor pattern maybe, for example, aluminum, titanium, chromium, tungsten, copper,molybdenum, gold, and/or platinum. Optionally, one or more layers ofother materials may be disposed below (i.e. between the conductor layerand the piezoelectric plate), and/or on top of the conductor pattern.For example, a thin film of titanium, chrome, or other metal may be usedto improve the adhesion between the conductor pattern and thepiezoelectric plate. A conduction enhancement layer of gold, aluminum,copper or other higher conductivity metal may be formed over portions ofthe conductor pattern (for example the IDT busbars and interconnectionsbetween the IDTs). Only the busbars of the IDT can be formed on theinsulating layer, or, alternatively, other portions or all of theconductor pattern can be formed on the insulating layer.

The conductor pattern may be formed at 730 by depositing one or moremetal layers over the surface of the piezoelectric plate and/or theinsulating layer. The excess metal may then be removed by etchingthrough patterned photoresist. The conductor pattern can be etched, forexample, by plasma etching, reactive ion etching, wet chemical etching,and other etching techniques.

Alternatively, the conductor pattern may be formed at 730 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorpattern and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 740, a front-side dielectric layer may be formed by depositing one ormore layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In a second variation of the process 700, one or more cavities areformed in the back side of the substrate at 710B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 700, a back-side dielectric layermay be formed at 750. In the case where the cavities are formed at 710Bas holes through the substrate, the back-side dielectric layer may bedeposited through the cavities using a conventional deposition techniquesuch as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 700, one or more cavities in theform of recesses in the substrate may be formed at 710C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device.

In all variations of the process 700, the filter device is completed at760. Actions that may occur at 760 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 760is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front sideof the device. After the filter device is completed, the process ends at795.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A filter device comprising: a substrate having asurface; a single-crystal piezoelectric plate having front and backsurfaces, the back surface attached to the surface of the substrate,portions of the piezoelectric plate forming one or more diaphragmsspanning respective cavities in the substrate; and a conductor patternformed on the front surface, the conductor pattern including a pluralityof interdigital transducers (IDTs) of a respective plurality of acousticresonators, interleaved fingers of each of the plurality of IDTsdisposed on one of the one or more diaphragms of the piezoelectricplate, wherein an insulating layer is disposed between the piezoelectricplate and portions of the conductor pattern other than the interleavedfingers.
 2. The device of claim 1, wherein the piezoelectric plate andinterleaved fingers are configured such that a radio frequency signalapplied to the plurality of IDTs excites a primary shear acoustic modein the one or more diaphragms.
 3. The device of claim 1, wherein thepiezoelectric plate is one of lithium niobate and lithium tantalate. 4.The device of claim 3, wherein the piezoelectric plate is one of Z-cut,rotated Z-cut, and rotated YX-cut.
 5. The device of claim 1, wherein theinsulating layer is comprised of one or both of SiO₂ and Si₃N₄.
 6. Thedevice of claim 5, wherein the insulating layer has a thickness in arange between 1 nm and a thickness of the conductor pattern.
 7. Thedevice of claim 1, wherein the insulating layer does not extend acrossthe cavities.
 8. The device of claim 1, wherein the portions of theconductor pattern comprise at least one busbar, and the insulating layeris between the at least one busbar and the piezoelectric plate.
 9. Thedevice of claim 8, wherein the insulating layer is configured to reduceacoustic coupling between the at least one busbar and the piezoelectricplate.
 10. The device of claim 1 further comprising a front-sidedielectric formed on the front side of the piezoelectric plate betweenthe interleaved fingers.
 11. A method of fabricating a filter devicecomprising: attaching a piezoelectric plate to a substrate, thepiezoelectric plate having front and back surfaces, the back surfaceattached to the substrate, portions of the piezoelectric plate formingone or more diaphragms spanning respective cavities in the substrate;forming an insulating layer on the front surface of the piezoelectricplate; and forming a conductor pattern comprising a plurality ofinterdigital transducers (IDTs) of a respective plurality of acousticresonators on the piezoelectric plate, interleaved fingers of each ofthe plurality of IDTs on the one or more diaphragms of the piezoelectricplate, wherein the insulating layer is disposed between thepiezoelectric plate and portions of the conductor pattern other than theinterleaved fingers.
 12. The method of claim 11, wherein thepiezoelectric plate and interleaved fingers are configured such that aradio frequency signal applied to the IDT excites a primary shearacoustic mode in the diaphragm.
 13. The method of claim 11, wherein thepiezoelectric plate is one of lithium niobate and lithium tantalate. 14.The method of claim 13, wherein the piezoelectric plate is one of Z-cut,rotated Z-cut, and rotated YX-cut.
 15. The method of claim 11, whereinthe insulating layer is comprised one or both of SiO₂ and Si₃N₄.
 16. Themethod of claim 15, wherein the insulating layer has a thickness in arange between 1 nm and a thickness of the conductor pattern.
 17. Themethod of claim 11, wherein the insulating layer does not extend acrossthe cavity.
 18. The method of claim 11, wherein the conductor patterncomprises at least one busbar, and the insulating layer is between theat least one busbar and the piezoelectric plate.
 19. The method of claim18, wherein the insulating layer is configured to reduce acousticcoupling between the busbar and the piezoelectric plate.
 20. The methodof claim 10 further comprising forming a front-side dielectric on thefront side of the piezoelectric plate between the interleaved fingers.